Tire manufacturers put a lot of time and money into product research. They know best what the inflation level should be and they put that number on the side of the tire. If it says maximum inflation is 105 psi cold, inflate it to 105 psi cold. They do not put a minimum psi on the tire. The maximum level is the level to bring the tire without over-inflating it. Bring it to that point and keep it there.
Finding tires inflated to a lower rate as you did could be explained a couple ways.
It may be that the tires were inflated to the correct level but they were hot at the time. That would result in a lower level when the tires cooled. (The air in the tire expands and contracts as the temperature inside the tire increases and decreases. You can see this yourself by checking the pressure on a cold tire and checking it again while running on a hot day.)
The tires may have been properly inflated but the air simply leaked out over time. That is normal and it is the reason why you should regularly check and reinflate your tires.
The dealer may have intentionally deflated the tires to the level you found them. Some people believe softer tires produce a softer ride. A soft ride is a desirable condition if you are a sales rep trying to move a truck off off your lot. It is an undesirable condition if you are the one who's life is at stake when you are driving the truck down the road.
I have been advised by more than one seasoned driver to run under-inflated tires on the front to provide a smoother ride. No thank you.
As Broompilot correctly points out, heat kills tires. An underinflated tire flexes more than a properly inflated one. More flexing means more heat and also shortened life of the belts and lining inside the tire. That can translate to a blowout and life-threatening situation.
An over-inflated tire will buldge out the center of the the tread and lead to premature wear. Over-inflated tires can be easily identified by looking at the tread. If the center treads are worn deeper than the outer ones, the tire is likely over-inflated. This can be seen by the naked eye and felt by running your fingers over the treads on tires that have been over-inflated for a long time. A tread-depth guage can pick this up earlier.
Even for drivers with no mechanical ability, tire care is easily done and well worth the effort. A tire guage, tread-depth guage and the willingness to check and inflate the tires every so often (a week to ten days) will help keep you safe on the road and greatly enhance tire life. Keeping your eye on tire wear can help you identify early any front suspension troubles that may be developing.
It is normal for certain front-end components to wear out and be replaced. Catching them early helps keep you safe and saves you money. You don't need to know about caster, camber, toe, ball joints and such to know there may be problems with them developing in your truck. Knowing about tire wear is enough to tell you should take your truck in to the shop.
Even if you are not mechanically inclined, with a little bit self-education and an investment in a couple inexpensive tools (tire guage, tread-depth guage), you can greatly increase your tire life and safety on the road. It will give you a self-confidence boost as a driver too.
While drivers can't know everything about truck mechanics, tires are easy to learn about and care for. The payoff of doing so is immediate and the safety benefits are significant.
Tire manufacturers provide good information about tires. For a start, see:
http://www.michelintruck.com/michelintruck/other/Reference.jsp
http://www.goodyear.com/truck/tireinfo/servicemanual.html
As you may have guessed by now, I am a fanatic about tire maintenance on our truck. I own a tire guage and tread-depth guage. When we built our straight truck I added a gladhand. It is mounted under the reefer body half-way between the front and rear tires. For those who do not know, a gladhand is the connector used on semi trucks to connect the air lines between the tractor and trailer.
Most truck stops sell an air hose with a gladhand connection on one end and a tire chuck on the other. With this hose, the compressor that is used with air brakes can also be used to inflate your truck tires. Straight trucks do not have the air lines semi trucks have, so I added a gladhand.
That addition makes inflating tires almost fun. It frees you from having to first find a tire hose that works at a truck stop fuel island and then feeling bad as the hurried trucker behind you in line waits while you inflate your tires. Or, at TA truck stops, it saves you the trouble of requesting a release form to sign before they give you the air hose to use, and the dirty look from the shop tech who would rather not be troubled to go get the hose, and the time it takes to move your truck to access the hose.
With the gladhand and an air hose of my own, I can inflate my tires anywhere I can park and idle the truck (to run the compressor).
A gladhand can be added to any straight truck that has an air compressor. I highly recommend it for any owner-operator who seeks to maximize his or her tire life and safety, and also be free of truck stop air-hose hassles.
I have also reduced my tire-maintenance effort with two other items. Valve stem extenders on the rear wheels make it easier to access the inner dual tire vale stems. Valve stem caps that allow you to check and inflate the tires without removing the caps are a big time saver.
Those little caps are great. If you don't have to remove the valve stem cap to check your tire pressure, checking your pressure becomes a simple matter of walking around the truck and pressing your tire guage onto the stem. You can check all tires in very little time with very little effort.
With a gladhand, tire hose, tire guage, tread-depth guage, valve stem extenders and easy-access valve stem caps, I have reduced my tire care time to a few minutes a week.
